Presently, as rare earth magnets, two kinds of magnets, i.e., rare earth/cobalt-based magnets and rare-earth/iron/boron-based magnets are widely used in various fields.
Among them, the rare-earth/iron/boron-based magnets (hereinafter, referred to as “R-T-(M)-B magnets”, R is a rare earth element including Y, T is Fe or a mixture of Fe with Co and/or Ni, M is an additive element (at least one of Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, W, for example), and B is boron or a mixture of boron with carbon) exhibit the highest maximum magnetic energy product in various kinds of magnets, and they are relatively cheep. For these reasons, the R-T-(M)-B magnets are actively adopted to various electronic equipments.
For example, in order to attain high energy efficiency, or for the purposes of miniaturization, weight reduction, and the like, the rare earth magnets are used in various rotating machines (such as motors). For example, Japanese Laid-Open Patent Publication No. 2000-41372 discloses a moving-magnet type direct-current torque motor 10 as shown in FIG. 5.
The direct-current torque motor 10 includes a rotor 1 in which permanent magnets 5 and 6 are cylindrically fixed around a cylindrical iron core (soft iron, for example) 4 having an axis 3 as the center, and a stator 2 having a substantially “C” shape in which a first magnetic pole portion 7 and a second magnetic pole portion 8 disposed oppositely while keeping a predetermined gap with the rotor 1 in end portions thereof, respectively, are formed, and a coil 9 is wound around a base portion (in an upper portion of the figure). The stator 2 is formed by superposing flat rolled magnetic steel sheets and strip having a saturation magnetic flux density of 1.6 T (tesla) or more, for example.
The permanent magnets 5 and 6 disclosed in the above-identified publication are permanent magnets each having a semi-cylindrical shape (segment), and are magnetized in directions opposite to each other. For example, in the magnet 5, a front face (an outer circumference) side is the N pole, and a back face (an inner circumference) side is the S pole. On the other hand, in the magnet 6, a front face (an outer circumference) side is the S pole, and a back face (an inner circumference) side is the N pole. Instead of the magnets 5 and 6, a cylindrical magnet (not shown) which is integrally compacted can be used.
It was found that when the magnets 5 and 6 used in a rotor of the above-mentioned motor 10 were magnetized in a condition where they were disposed in a cylindrical manner, or an integrally formed cylindrical magnet was magnetized, a conventional magnetization in which a magnetic field was applied once in a diametrical direction caused a problem that a boundary portion between regions magnetized in directions opposite to each other (corresponding to the magnets 5 and 6 in FIG. 5) was not sufficiently magnetized. In such a magnet which is insufficiently magnetized, an average magnetic flux density is reduced and an angle range in which a desired surface magnetic flux density is obtained is narrowed. In addition, in an angle distribution (around an axis) of the surface magnetic flux density, an extremal value occurs in the vicinity of the boundary portion, and a linearity of the variation in the surface magnetic flux density is lowered. As a result, it may be difficult to adjust the angle of a rotating machine with good precision, or the performance of the rotating machine may not be sufficiently exercised.